1. Field of the Disclosure
The present disclosure relates to a transformer for reducing eddy current losses of a coil, and particularly, to a transformer in which a cut part which is provided by removing a portion of a conductor is provided in each of an upper end and a lower end of a coil, and thus, eddy current losses caused by leakage flux are reduced.
2. Background of the Disclosure
Generally, a high voltage power transformer is an electronic device that is provided in a power system and is supplied with a voltage from a power station to boost and lower the voltage. The high voltage power transformer plays an important role in transmitting power to a power consumer.
Electrical loss occur in operating and managing a transformer, and a loss of a transformer is represented by a sum of no load loss, load loss, and loss which occurs in an auxiliary device (a fan, a pump, and/or the like). The no load loss is loss which occurs in a core configuring a transformer, and the load loss is loss which occurs in a coil, a supporting structure near the coil, a tank, and/or the like.
In the load loss, most of losses are direct current (DC) resistance losses caused by a coil resistance, but stray load loss and eddy current loss which occurs in a coil, a supporting structure, and a tank due to leakage flux cannot be neglected. In particular, eddy current loss which occurs in a coil causes local overheating, and is a factor that largely affects long-time operation reliability of a transformer.
FIG. 1 schematically illustrates a cross-sectional view of a core and a coil of a related art transformer. FIGS. 2A to 2C illustrate examples of a conductor applied to a coil of the related art transformer. Here, FIG. 2A illustrates a flat conductor 4a, FIG. 2B illustrates a double conductor 4b, and FIG. 2C illustrates a transposed conductor 4c. FIG. 3 illustrates a detailed cross-sectional view of a coil of the related art transformer.
In the related art transformer, as illustrated in FIG. 1, a plurality of coils 2 and 3 are disposed to form a concentric circle with respect to a coil 1. In FIG. 1, only two coils 2 and 3 are illustrated, but the number of coils may be two or more depending on a place where the transformer is used.
The coils 2 and 3 of the transformer are manufactured by winding the conductors 4 formed of copper, aluminum, and an alloy. The conductors 4 are surrounded by an insulating material 5, for insulating turns of the conductors 4. FIG. 3 illustrates various forms of conductors used for the coils 2 and 3 of the transformer. An obliquely-striped portion refers to the conductor 4, and a portion surrounding the conductor 4 refers to an insulating material.
Referring to FIG. 3, the coil 2 of the transformer is configured by a combination of several pieces of conductors (for example, a combination of sections 2a, 2b, 2c, . . . where the transposed conductor 4c is wound). In order to cool the coil 2, a vertical cooling duct 7 may be provided in the coil sections 2a, 2b, 2c, . . . . FIG. 3 illustrates a coil where the transposed conductor 4c is used and two vertical cooling ducts 7 are applied for cooling the coil 2.
FIG. 4 illustrates leakage flux of a transformer coil and eddy current loss of a coil end caused by the leakage flux. When power is applied to a primary coil 2 of a transformer, a voltage is induced to a secondary coil 3, and a direction of a current flowing in the secondary coil 3 is opposite to that of a current flowing in the primary coil 2. Due to such an influence, leakage flux largely occurs between the primary coil 2 and the secondary coil 3, and thus, an eddy current is generated near each of the coils 2 and 3, for attenuating the leakage flux. Loss caused by the eddy current is referred to as eddy current loss. The eddy current loss is affected by a level and a direction of leakage flux of a transformer, a dimension of a coil conductor, a current density of a coil, a resistance of a coil, and a power frequency. A portion where a number of eddy current losses caused by leakage flux occurs is an end of a transformer coil, and a local temperature rise (hot-spot) caused by eddy current loss is high measured. Referring to FIG. 4, an eddy current loss of a coil section 3a is larger than that of a coil section 3b. 
As described above, examples of factors affecting the magnitude of eddy current loss which occurs in a coil include a maximum value of leakage flux, an incident direction of leakage flux with respect to a coil, a dimension based on a shape and a size of a coil conductor, a current density of a coil, a resistance of a conductor, and a level of a power frequency. In these factors, a factor for satisfying characteristic (% impedance, a capacity, and/or the like) required by a transformer and adjusting eddy current loss is the dimension of the coil conductor (a, a′, a″, or be in FIG. 2). Generally, in order to reduce eddy current losses of a coil, the double conductor 4b is used instead of the flat conductor 4a, and the transposed conductor 4c is used instead of the double conductor 4b. In coil dimensions, a dimension (a) which the most largely affects an eddy current loss of a coil is illustrated in FIG. 2.
Therefore, the transposed conductor 4c where the dimension (a) of the coil conductor is the smallest is used for reducing eddy current losses.
However, in the related art, a method of reducing eddy current losses by adjusting a dimension of a coil conductor has a drawback which is difficult to apply for maintaining mechanical strength. Also, it is required to maintain an appropriate current density of a coil, and thus, a dimension (b) of the conductor should be relatively enlarged for decreasing the dimension (a) of the conductor. For this reason, eddy current loss caused by the dimension (b) increases.